1. Field of the Invention
The present invention relates to an engine control device for use in an internal combustion engine for performing a variety of controls such as ignition timing control, fuel injection timing control, etc., based on reference crank angles.
2. Description of the Prior Art
In an internal combustion engine, when the ignition timing is advanced or retarded or the fuel injection timing is advanced or retarded based on reference crank angles, it is usual to use a method of controlling by performing a predictive computation of the time required for reaching the target angle from the reference crank position based on the engine's rotational period. However, this method involves the following difficulties.
In the aforementioned method of performing predictive computation based on the rotational period, since the time from the latest reference crank angle signal to the necessary timing (e.g., the target ignition timing) is estimated based on the engine's rotational frequency (the time of one revolution) between the reference crank angle signal prior to the necessary timing and the reference crank angle signal one revolution before the former, the estimated value will be unstable with respect to the rotational variations (rotational changes) of the crankshaft.
In multi-cylinder engines, rotational variations occur due to combustion variations depending upon individual cylinders, which are attributed to intake amount scatter, the scatter in the sprayed amount of injected fuel, variation in the injector characteristics dependent on individual cylinders, variation in the carburetor characteristics dependent on individual cylinders. In particular, in the very low speed range in which lower amounts of fuel and intake air are used, the ratios of the above variations in the required amount of fuel and the amount of intake air become large and the rotational inertia is low, so that a slightest fluctuation in combustion for each cylinder may significantly affect the variations in rotation.
In a typical case where predictive computation is performed based on the rotational period of one revolution, for a four cycle engine, the period of one revolution (FIG. 1) for determining the target angle is affected by the combustion of other cylinders, hence the precision of the predictive computation is low in the very low rotational range where combustion fluctuations dependent on individual cylinders are liable to occur as stated above.
In the above predictive computing method, #1 ignition timing is controlled on the premise that the average rotational frequency during the period between #3.alpha. and the previous #3.alpha. is approximately equal to the average rotational frequency during the period from #3.alpha. to the ignition timing, as is shown in FIG. 2, for example. In this case, the one revolution roughly corresponds to the combustion stroke of cylinder #3 and the compression and combustion strokes of cylinder #2. Since the average rotational frequency during this interval is used to estimate the average rotational frequency for the compression stroke of cylinder #1, the estimate naturally presents poor precision if there are variations in combustion dependent on individual cylinders.